A method for improving three-way catalyst (TWC) performance by superimposing a low frequency lean air-to-fuel ratio (AFR)bias perturbation onto the standard AFR oscillations is described. This observation of Catalyst Regeneration (CatRegen) has been attributed to a reactivation of poisoned precious metal sites on the catalyst surface.Preliminary tests under steady-state conditions show that there is a gradual reduction in TWC activity for NOx after a lean-rich transition, suggesting a temporary poisoning of the active precious metal sites on the TWC under rich conditions. This deactivation can be prevented by periodically exposing the catalyst to lean exhaust gas; which has led to the development of the CatRegen system. Experiments conducted on a 1.6 litre engine with a European Stage IV intent Pd/Rh starter and Pd/Rh underfloor TWC combination illustrate that the emissions of THC, CO, and particularly NOx can be significantly reduced over the ECE Stage III drive cycle using the CatRegen system. Further studies to investigate the reversible nature of the catalyst deactivation, using low sulfur and sulfur-doped gasoline fuel, implicate fuel sulfur as the temporary poison. It would appear that the low frequency lean bias perturbations temporarily reactivate the Pd-sites of a Pd/Rh TWC by oxidising reduced sulfur species from the catalyst surface.